The invention relates to a clamping circuit, and more specifically to a clamping circuit for restoring the DC level of video signals.
Video signals are processed in many different applications including televisions, video capture equipment, VCRs and camcorders. When transmitting video signals, the D.C. reference level thereof may be lost. Therefore, a clamping circuit is generally built into video receivers to restore the DC level of video signals to a reference level within the supply range of the video receivers.
FIG. 1A is a schematic diagram of a conventional clamping circuit 10, having a coupling capacitor C1, two current sources 14 and 16, and a comparator 18. The coupling capacitor C1 receives a video input signal Vin to be clamped. The comparator 18 coupled between the coupling capacitor C1 and the second current source 16, receives the video input signal Vin via the coupling capacitor C1 at a positive input terminal thereof and a reference voltage Vref at a negative input terminal thereof, thereby generating a charging control signal. Moreover, the current source 14 is coupled between the coupling capacitor C1 and ground, and the current source 16 coupled between a power line Vcc and the coupling capacitor C1, receives the charging control signal from the comparator 18. When receiving the video input signal Vin, the comparator 18 compares the magnitude of the video input signal Vin to that of the reference voltage Vref, and generates the charging control signal in accordance with the comparison result to the second current source 16. For example, when the AC-coupled signal Vac is less than the reference voltage Vref, the comparator 18 generates the charging control signal in a LOW state (i.e. logic ‘0’), turning on the current source 16, thereby charging the coupling capacitor C1 and raising the level of the AC-coupled signal Vac. When the video input signal Vin exceeds the reference voltage Vref, the current source 16 is turned off with the charging control signal in a HIGH state (i.e. logic ‘1’). The current source 14 is a weak current source, continuously discharging the coupling capacitor C1, lowering the level of the video input signal Vin slowly. FIGS. 1B and 1C are exemplary waveforms of the video input signal Vin before and after being clamped by the clamping circuit 10 respectively. Comparing FIG. 1B with FIG. 1C, it can be found that the lowest level of the video input signal Vin is clamped to the predetermined reference voltage level Vref by the clamping circuit 10.
The conventional clamping circuit, however, requires an extra current source such as current source 16 to charge the coupling capacitor; this may increase the required chip area and cost.